I've yet to meet someone who preferred reading a book on their computer, or on a small PDA screen. Ebook readers are far too expensive, and they only do one thing. The restrictions placed on most eBooks, in terms of how you can use them and on how many computers, are also a little prohibitive to their expansion.
Music sharing, on the other hand, is different. A user can download an entire CD, burn it to a disc, and the only thing they're missing is the CD art and sleeve. Or they can listen to it on their $50 MP3 player; share them with friends, etc. So while there's incentive to buy CD's if you want to support the band (something which won't go away) or get the jewel case art, you can get the real substance of a CD quick, and take it anywhere.
So, while eBooks and MP3's are both electronic innovations on old technology (eg: fruit) they're completely different mediums (apples and oranges), making it difficult to draw an accurate comparison.